MML Series Investment Fund N-CSRCertified annual shareholder report (management investment company)

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UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

FORMN-CSR

CERTIFIED SHAREHOLDER REPORT OF REGISTERED

MANAGEMENT INVESTMENT COMPANIES

  Registrant’s telephone number, including area code:      (860) 562-1000                             

  Date of fiscal year end:      12/31/2019                            

  Date of reporting period:      12/31/2019                          

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Item 1. Reports to Stockholders.

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This material must be preceded or accompanied by a current prospectus (or summary prospectus, if available) for the MML Series Investment Fund. Investors should consider a Fund’s investment objective, risks, and charges and expenses carefully before investing. This and other information about the investment company is available in the prospectus (or summary prospectus, if available). Read it carefully before investing.

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Monitor your asset allocation regularly – and diversify*

The most common retirement plan investments – stocks, bonds, and short-term/money market investments – typically (although not always) behave differently from one another at any given time. Each of these asset classes contains an array ofsub-categories, such assmall-cap stocks, international stocks, and high-yield bonds. Many financial professionals believe that investors can reduce the risk of over-exposure to one or two poorly performing investment types by investing in a diverse mix of investment types andsub-categories.

Choose the investments you feel are right for you – and seek professional guidance

Many financial advisors believe that retirement savers who select an appropriate combination of investments that aligns with both their retirement income goals and how they feel about investing can help them withstand the inevitable ups and downs of the markets.

If you work with a financial professional, you may wish to consider reaching out to him or her for assistance in helping you ensure:

We’re people protecting people

MassMutual believes that your long-term financial security is crucial. Our ongoing commitment is to help people financially protect their families and put themselves on the path to a more secure retirement. And that’s why we continually remind retirement investors of the importance of maintaining a long-term perspective on retirement planning. Our view is that planning isn’t about crunching numbers – it’s about those moments when it all pays off – and that the more you plan for life’s greatest moments, the more you can relax and enjoy them. Thank you for your continued confidence and trust in MassMutual.

Sincerely,

Eric Wietsma

President

^© 2020 Massachusetts Mutual Life Insurance Company (MassMutual), Springfield, MA. All rights reserved. www.massmutual.com Underwriter: MML Distributors, LLC. (MMLD) Member FINRA and SIPC (www.FINRA.org and www.SIPC.org), 100 Bright Meadow Blvd., Enfield, CT 06082. MMLD is a wholly-owned subsidiary of MassMutual. Investment advisory services provided to the Funds by MML Investment Advisers, LLC (MML Advisers), a wholly-owned subsidiary of MassMutual. The information provided is the opinion of MML Advisers as of 1/1/20 and is subject to change without notice. It is not to be construed as tax, legal, or investment advice. Of course, past performance does not guarantee future results.

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December 31, 2019

Market Highlights

Market Environment

U.S. equity investors benefited from a decrease in volatility in 2019 and expectations that a global recession would be avoided in the near term. U.S. stocks delivered positive returns in each of the four quarters of the year. The S&P 500^® Index* (the “S&P 500”) and the Dow Jones Industrial Average^SM (the “Dow”) each posted strong positive results for the year, gaining 31% and 25% respectively.

In the first quarter of 2019, investors pushed aside concerns over slowing global economic growth and increased market volatility to push global stock prices to nearall-time highs. The market rally was fueled by the Fed’s decision to hold off on interest rate hikes in 2019 (and to instead lower rates), the end of the government shutdown (which ran from December 22, 2018 to January 25, 2019), and evidence that the trade war between the U.S. and China was on a path to resolution.

Markets continued to gain traction in the second quarter, as investors tamped down fears of a global economic slowdown, and continued to send global stock prices on an upward trajectory. The market rally was bolstered by expectations that the Fed would be cutting interest rates as early as July 2019 and the belief that the trade negotiations between the U.S. and China remained on track.

Volatility returned to the markets in the third quarter, as investor concerns that tradewar-related conflicts would spark a global economic slowdown flew in the face of strong consumption in the United States, a rebound in U.S. housing, and renewed monetary stimulus from a majority of the world’s central banks. The temporary inversion of the U.S. yield curve also troubled investors. Inversions of the yield curve, where short-term rates are higher than long-term rates, have historically signaled future economic contractions.

It was back to the races in the fourth quarter, with investors responding positively to the Fed’s move to lower the federal funds rate (i.e., the interest rate that banks and financial institutions charge each other for borrowing funds overnight), better-than-expected corporate earnings, the resurgence of the housing sector in the U.S., and progress being made in the U.S.-China trade war. Emerging-market stocks led global markets upward in the face of a weakening U.S. dollar.

At year end, the broad market S&P 500 ended up 31.49% and the Dow followed a similar pattern, advancing 25.34%. The technology-heavy NASDAQ Composite Index gained 36.69%, led by technology giant Apple, which appreciated 87% for the year.Small-cap stocks underperformed their larger peers and growth stocks continued to outperform their value counterparts during the period.

All sectors of the S&P 500 delivered positive results for the year, with the information technology sector being the clear winner, up 50.29%. The information technology, communication services, and financials sectors were the only areas of the market that outperformed the broad market, as represented by the S&P 500. The energy sector, while up, was the top laggard, up just 11.81% for the year.

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Developed international markets, as measured by the MSCI EAFE^® Index, had a strong year, although they trailed their U.S. peers, and ended the year 22.01% higher. Emerging markets, as measured by the MSCI Emerging Markets Index, also had a strong year, with an advance of 18.42%. International stocks benefited from accommodative monetary policy, investors’ belief that the U.S.-China trade war was done escalating, and diminished concerns that the global economy was on the brink of a recession.

The Fed’s influence on markets was considerable in 2019. In the first quarter, it signaled that it was done raising interest rates over concerns that higher rates were impacting economic expansion. In the second quarter, Fed Chairperson Jerome Powell addressed fears of how the continuing U.S.-China trade war could hurt the U.S. economy and commented that the Fed would “act as appropriate to sustain the expansion.” This perceived willingness to cut interest rates was positive for risk assets and drove equities higher. In the second half of 2019, the Fed took action to sustain the economic expansion, cutting interest rates in July, September, and October.

Bond yields fell during the year, with the10-year U.S. Treasury bond falling below 1.50% in September. After starting the year at 2.69%, yields fell 0.77% to end 2019 at 1.92%. Since falling yields drive bond prices up, returns on the Bloomberg Barclays U.S. Aggregate Bond Index were strong for the year, with an 8.72% advance. Investment-grade corporate bonds also fared well in the falling yield environment. The Bloomberg Barclays U.S. Corporate Bond Index, which tracks investment-grade corporate bonds, ended the year with a gain of 14.54%. The Bloomberg Barclays U.S. Corporate High Yield Index also ended the year in positive territory, appreciating 14.32%.

As we enter 2020, we continue to see high valuations in the equity market, from a historical perspective. At MassMutual, we remain convinced that a broadly diversified portfolio aligned with a long-term strategy is the wisest course for those investors seeking retirement income. We remind you that it is important to maintain perspective and have realistic expectations about the future performance of your investment portfolio. As described in this report, financial markets can reverse suddenly, stumble along sideways, and reverse again. As always, we recommend that you work with a personal financial professional, who can help you define an investment strategy that aligns with your comfort level with respect to markets like these, taking into consideration how long you have to save and invest, as well as your financial goals and risk tolerance. Thank you for your confidence in MassMutual.

The information provided is the opinion of MML Investment Advisers, LLC as of 1/1/20 and is subject to change without notice. It is not to be construed as tax, legal, or investment advice. Of course, past performance does not guarantee future results.

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What are the investment approaches of the Funds that constitute the MML Allocation Series (the “Series”), and who is the Series’ investment adviser?

The MML Allocation Series comprises five Funds – each of which has a “fund of funds” structure. The five Funds in the Series are MML Conservative Allocation Fund, MML Balanced Allocation Fund, MML Moderate Allocation Fund, MML Growth Allocation Fund, and MML Aggressive Allocation Fund.

Each Fund seeks to achieve as high a total return over time as is considered consistent with prudent investment risk, preservation of capital, and recognition of the Fund’s stated asset allocation. Each Fund invests in a combination of U.S. domestic and international mutual funds (“MML Underlying Funds”) using an asset allocation strategy. MML Underlying Funds will include a combination of series of the MML Series Investment Fund and MML Series Investment Fund II (advised by MML Investment Advisers, LLC (MML Advisers), a wholly-owned subsidiary of Massachusetts Mutual Life Insurance Company (MassMutual)), and may also include other,non-affiliated mutual funds. The MML Underlying Funds may invest in various asset classes, including equity securities, fixed income securities, and money market instruments. The Series’ investment adviser is MML Advisers.

Each Fund will normally invest most of its assets in MML Underlying Funds advised by MML Advisers; each Fund will normally invest no more than 10% of its assets in mutual funds not advised by MML Advisers (referred to here as“non-affiliated” funds).

Each Fund’s assets are allocated to its MML Underlying Funds according to an asset allocation strategy stated below.

How did each Fund perform during the 12 months ended December 31, 2019?

* Benchmark return

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All of the MML Allocation Series Funds underperformed their respective Custom Index as well as the Russell 3000 Index. Only the MML Aggressive Allocation Fund outperformed the MSCI ACWI ex USA Index, but all five funds in the Series outperformed the Bloomberg U.S. Aggregate Bond Index for the full year.

MML Allocation Fund Series Custom Indexes

For a discussion on the economic and market environment during the12-month period ended December 31, 2019, please see the Economic and Market Review, beginning on page 3.

Investment adviser discussion of factors that contributed to the Funds’ performance

For the year ended December 31, 2019, the Series Funds delivered positive double-digit returns for the year, as stocks outperformed fixed-income investments. Consequently, those Series Funds with higher equity allocations outperformed their Series counterparts with higher fixed-income allocations.

Stock markets across the globe were up significantly in 2019, as were the more credit- and interest rate-sensitive fixed-income markets. Allocations across various asset classes intended to diversify risk in the Funds delivered mixed results. The allocation to alternative asset classes detracted from performance, as these investments underperformed the global stock market, while the allocation to international and emerging-market stocks also hampered performance, asnon-U.S. stocks underperformed U.S. stocks in the face of a weakening global economy.

Manager selection of underlying Funds contributed to the Funds’ overall performance in 2019. Strong performance by MML International Equity Fund, MML Global Fund, MML Strategic Emerging Markets Fund, and MML Managed Bond Fund helped the Funds’ overall full-year returns. Weak results from MML Foreign Fund and MML Fundamental Value Fund significantly detracted from the Funds’ performance.

Investment adviser outlook

As we enter 2020, Fund management views the stock markets as extended and valuations high, relative to historical levels. Although the economy has continued to grow modestly, Fund management expects that growth could remain muted in 2020 and beyond, though they believe that the likelihood of recession remains low. Several factors that support their views include the fact that we are late in a typical economic growth cycle, there are diminishing tailwinds from a declining unemployment rate, company profit margins remain above long-term trends, and corporate debt levels are back nearall-time highs. Fund management believes that more accommodative monetary policy from the U.S. Federal Reserve Board or fiscal policy from state and federal governments could extend the economic cycle. Against this backdrop, Fund management believes that diversification across global assets could be a smart approach for what they believe could be a bumpy ride in 2020.

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GROWTH OF $10,000 INVESTMENT - INITIAL CLASS

The graph above illustrates a representative class of the Fund’s historical performance for the past 10 fiscal years or since inception (for Funds lacking 10-year records) in comparison to its benchmark index, as well as one or more additional indexes, if applicable. The performance of other share classes will be greater than or less than the class depicted above.

* Benchmark

Generally accepted accounting principles require adjustments to be made to the net assets of the Fund at period end for financial reporting purposes only, and as such, the total return based on the unadjusted net asset value per share may differ from the total return reported in the financial highlights.

Performance results in the graph and table reflect the percentage change in net asset value, including reinvestment of dividends and capital gains distributions. Investors should note that the Fund is a professionally managed mutual fund, while the index or indexes shown above are unmanaged, cannot be purchased directly, and, with the exception of any peer group index, do not incur expenses.

Performance data quoted in the graph and table represents past performance; past performance is not predictive of future results. The investment return and principal value of shares of the Fund fluctuate, so your shares, when sold, may be worth more or less than their original cost. Current performance of the Fund may be lower or higher than the performance quoted. Performance data current to the most recent month end may be obtained by calling 1-888-309-3539.

Investors should read the Fund’s prospectus with regard to the Fund’s investment objectives, risks, and charges and expenses in conjunction with these financial statements. The Fund’s return reflects changes in the net asset value per share without the deduction of any product charges (e.g., cost of insurance, mortality and expense risk charges, administrative fees, and CDSL). The inclusion of these charges would have reduced the performance shown here.

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GROWTH OF $10,000 INVESTMENT - INITIAL CLASS

The graph above illustrates a representative class of the Fund’s historical performance for the past 10 fiscal years or since inception (for Funds lacking 10-year records) in comparison to its benchmark index, as well as one or more additional indexes, if applicable. The performance of other share classes will be greater than or less than the class depicted above.

* Benchmark

Generally accepted accounting principles require adjustments to be made to the net assets of the Fund at period end for financial reporting purposes only, and as such, the total return based on the unadjusted net asset value per share may differ from the total return reported in the financial highlights.

Performance results in the graph and table reflect the percentage change in net asset value, including reinvestment of dividends and capital gains distributions. Investors should note that the Fund is a professionally managed mutual fund, while the index or indexes shown above are unmanaged, cannot be purchased directly, and, with the exception of any peer group index, do not incur expenses.

Performance data quoted in the graph and table represents past performance; past performance is not predictive of future results. The investment return and principal value of shares of the Fund fluctuate, so your shares, when sold, may be worth more or less than their original cost. Current performance of the Fund may be lower or higher than the performance quoted. Performance data current to the most recent month end may be obtained by calling 1-888-309-3539.

Investors should read the Fund’s prospectus with regard to the Fund’s investment objectives, risks, and charges and expenses in conjunction with these financial statements. The Fund’s return reflects changes in the net asset value per share without the deduction of any product charges (e.g., cost of insurance, mortality and expense risk charges, administrative fees, and CDSL). The inclusion of these charges would have reduced the performance shown here.

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GROWTH OF $10,000 INVESTMENT - INITIAL CLASS

The graph above illustrates a representative class of the Fund’s historical performance for the past 10 fiscal years or since inception (for Funds lacking 10-year records) in comparison to its benchmark index, as well as one or more additional indexes, if applicable. The performance of other share classes will be greater than or less than the class depicted above.

* Benchmark

Generally accepted accounting principles require adjustments to be made to the net assets of the Fund at period end for financial reporting purposes only, and as such, the total return based on the unadjusted net asset value per share may differ from the total return reported in the financial highlights.

Performance results in the graph and table reflect the percentage change in net asset value, including reinvestment of dividends and capital gains distributions. Investors should note that the Fund is a professionally managed mutual fund, while the index or indexes shown above are unmanaged, cannot be purchased directly, and, with the exception of any peer group index, do not incur expenses.

Performance data quoted in the graph and table represents past performance; past performance is not predictive of future results. The investment return and principal value of shares of the Fund fluctuate, so your shares, when sold, may be worth more or less than their original cost. Current performance of the Fund may be lower or higher than the performance quoted. Performance data current to the most recent month end may be obtained by calling 1-888-309-3539.

Investors should read the Fund’s prospectus with regard to the Fund’s investment objectives, risks, and charges and expenses in conjunction with these financial statements. The Fund’s return reflects changes in the net asset value per share without the deduction of any product charges (e.g., cost of insurance, mortality and expense risk charges, administrative fees, and CDSL). The inclusion of these charges would have reduced the performance shown here.

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GROWTH OF $10,000 INVESTMENT - INITIAL CLASS

The graph above illustrates a representative class of the Fund’s historical performance for the past 10 fiscal years or since inception (for Funds lacking 10-year records) in comparison to its benchmark index, as well as one or more additional indexes, if applicable. The performance of other share classes will be greater than or less than the class depicted above.

* Benchmark

Generally accepted accounting principles require adjustments to be made to the net assets of the Fund at period end for financial reporting purposes only, and as such, the total return based on the unadjusted net asset value per share may differ from the total return reported in the financial highlights.

Performance results in the graph and table reflect the percentage change in net asset value, including reinvestment of dividends and capital gains distributions. Investors should note that the Fund is a professionally managed mutual fund, while the index or indexes shown above are unmanaged, cannot be purchased directly, and, with the exception of any peer group index, do not incur expenses.

Performance data quoted in the graph and table represents past performance; past performance is not predictive of future results. The investment return and principal value of shares of the Fund fluctuate, so your shares, when sold, may be worth more or less than their original cost. Current performance of the Fund may be lower or higher than the performance quoted. Performance data current to the most recent month end may be obtained by calling 1-888-309-3539.

Investors should read the Fund’s prospectus with regard to the Fund’s investment objectives, risks, and charges and expenses in conjunction with these financial statements. The Fund’s return reflects changes in the net asset value per share without the deduction of any product charges (e.g., cost of insurance, mortality and expense risk charges, administrative fees, and CDSL). The inclusion of these charges would have reduced the performance shown here.

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GROWTH OF $10,000 INVESTMENT - INITIAL CLASS

The graph above illustrates a representative class of the Fund’s historical performance for the past 10 fiscal years or since inception (for Funds lacking 10-year records) in comparison to its benchmark index, as well as one or more additional indexes, if applicable. The performance of other share classes will be greater than or less than the class depicted above.

* Benchmark

Generally accepted accounting principles require adjustments to be made to the net assets of the Fund at period end for financial reporting purposes only, and as such, the total return based on the unadjusted net asset value per share may differ from the total return reported in the financial highlights.

Performance results in the graph and table reflect the percentage change in net asset value, including reinvestment of dividends and capital gains distributions. Investors should note that the Fund is a professionally managed mutual fund, while the index or indexes shown above are unmanaged, cannot be purchased directly, and, with the exception of any peer group index, do not incur expenses.

Performance data quoted in the graph and table represents past performance; past performance is not predictive of future results. The investment return and principal value of shares of the Fund fluctuate, so your shares, when sold, may be worth more or less than their original cost. Current performance of the Fund may be lower or higher than the performance quoted. Performance data current to the most recent month end may be obtained by calling 1-888-309-3539.

Investors should read the Fund’s prospectus with regard to the Fund’s investment objectives, risks, and charges and expenses in conjunction with these financial statements. The Fund’s return reflects changes in the net asset value per share without the deduction of any product charges (e.g., cost of insurance, mortality and expense risk charges, administrative fees, and CDSL). The inclusion of these charges would have reduced the performance shown here.

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What is the investment approach of MML American Funds Growth Fund, and who is the Fund’s investment adviser?

The Fund seeks to provide investors with growth of capital through a “master feeder” relationship. (See page 50 of this report for information about the “master feeder” relationship.) The Fund invests all of its assets in Class 1 shares of the American Funds Insurance Series – Growth Fund (the “Master Growth Fund”), a series of the American Funds Insurance Series, a registeredopen-end investment company, managed by Capital Research and Management Company (Capital Research) with substantially the same investment objective. The Master Growth Fund invests primarily in common stocks and seeks to invest in companies that appear to offer superior opportunities for growth of capital. The Master Growth Fund may invest up to 25% of its assets in common stocks and other securities of issuers domiciled outside the United States. The Master Growth Fund may hold a portion of its assets in cash or cash equivalents. The Fund’s investment adviser is MML Investment Advisers, LLC (MML Advisers).

How did the Fund perform during the 12 months ended December 31, 2019?

The Fund returned 30.22%, underperforming the 31.49% return of the S&P 500^® Index (the “benchmark”), which measures the performance of 500 widely held stocks in the U.S. equity market. The Fund significantly underperformed the 36.39% return of the Russell 1000^® Growth Index, which measures the performance of thelarge-cap growth segment of U.S. equity securities. It includes the Russell 1000 Index companies with higherprice-to-book ratios and higher forecasted growth values.

For a discussion on the economic and market environment during the12-month period ended December 31, 2019, please see the Economic and Market Review, beginning on page 3.

Capital Research’s discussion of factors that contributed to the Fund’s performance

In 2019, U.S. equities had their strongest annual gains since 2013, ending the year nearall-time highs. Stocks rose steadily, posting gains in 10 of 12 months. The U.S. trade war with China occasionally caused volatility, but the resilient U.S. economy and the Federal Reserve Board’s return to accommodative policies helped steer markets higher. Information technology companies posted the highest returns, with a gain of nearly 48% in aggregate. Outside the U.S., European stocks advanced, despite signs of slowing economic growth, a near-recession in Germany, and Brexit-related uncertainty in the U.K. that remained unresolved until the very last month of the year. (“Brexit” – an abbreviation for “British exit” – refers to the United Kingdom’s referendum to leave the European Union.) Emerging-market stocks advanced, overcoming bouts of volatility tied to global trade tensions, China’s economic slowdown, and a strong U.S. dollar.

For the year ended December 31, 2019, Fund holdings in the consumer discretionary and health care sectors detracted from the Fund’s relative performance. The Fund holding that was the top detractor from relative returns was information technology company Apple. The Fund was significantly underweight in Apple, relative to the benchmark, and the technology leader proved to be one of the top performers for the year.

Fund holdings in the communication services sector contributed to the Fund’s relative performance, led by social media giant Facebook, which was boosted by strong user and revenue growth.

Capital Research’s outlook

Although U.S. economic growth remained solid, Fund management is keeping a close watch on near-term economic indicators, such as wage growth, monetary policy, and trade relations. They continue to favor well-positioned, growth-oriented companies that are capable of generating free cash flow in myriad near-term economic environments – and remain optimistic that their firm’s global research will help Fund management identify attractive long-term investment opportunities.

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GROWTH OF $10,000 INVESTMENT - SERVICE CLASS I

The graph above illustrates the Fund’s historical performance for the past 10 fiscal years or since inception (for Funds lacking 10-year records) in comparison to its benchmark index, as well as one or more additional indexes, if applicable.

* Benchmark

Generally accepted accounting principles require adjustments to be made to the net assets of the Fund at period end for financial reporting purposes only, and as such, the total return based on the unadjusted net asset value per share may differ from the total return reported in the financial highlights.

Performance results in the graph and table reflect the percentage change in net asset value, including reinvestment of dividends and capital gains distributions. Investors should note that the Fund is a professionally managed mutual fund, while the index or indexes shown above are unmanaged, cannot be purchased directly, and, with the exception of any peer group index, do not incur expenses.

Performance data quoted in the graph and table represents past performance; past performance is not predictive of future results. The investment return and principal value of shares of the Fund fluctuate, so your shares, when sold, may be worth more or less than their original cost. Current performance of the Fund may be lower or higher than the performance quoted. Performance data current to the most recent month end may be obtained by calling 1-888-309-3539.

Investors should read the Fund’s prospectus with regard to the Fund’s investment objectives, risks, and charges and expenses in conjunction with these financial statements. The Fund’s return reflects changes in the net asset value per share without the deduction of any product charges (e.g., cost of insurance, mortality and expense risk charges, administrative fees, and CDSL). The inclusion of these charges would have reduced the performance shown here.

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What is the investment approach of MML American Funds International Fund, and who is the Fund’s investment adviser?

The Fund seeks long-term capital growth of assets through a “master-feeder” relationship. (See page 50 of this report for information about the “master feeder” relationship.) The Fund invests all of its assets in Class 1 shares of the American Funds Insurance Series – International Fund (the “Master International Fund”), a series of the American Funds Insurance Series, a registeredopen-end investment company managed by Capital Research and Management Company (Capital Research) with substantially the same investment objective. The Master International Fund invests primarily in common stocks of companies domiciled outside the United States, including companies domiciled in emerging markets, that Capital Research believes have the potential for growth. The Master International Fund may hold a portion of its assets in cash or cash equivalents. The Fund’s investment adviser is MML Investment Advisers, LLC (MML Advisers).

How did the Fund perform during the 12 months ended December 31, 2019?

The Fund returned 22.31%, outperforming the 21.51% return of MSCI All Country World Index (ACWI) ex USA (the “benchmark”), which measures the performance of the large- andmid-cap segments of the particular regions, excluding U.S. equity securities, including developed and emerging markets. The Fund outperformed the 22.01% return of the MSCI EAFE Index, which measures the performance of the large- andmid-cap segments of developed markets, excluding the U.S. and Canada equity securities.

For a discussion on the economic and market environment during the12-month period ended December 31, 2019, please see the Economic and Market Review, beginning on page 3.

Capital Research’s discussion of factors that contributed to the Fund’s performance

Global stocks generated the best returns in a decade in 2019, boosted by aggressive central bank stimulus measures and easing trade tensions toward the end of the year. U.S. stocks soared to near record highs in December, as the U.S. and China agreed to a limited trade war truce, with the aim of avoiding the imposition of new tariffs and other protectionist measures. Information technology stocks posted the highest returns, with a gain of nearly 48% in aggregate. Outside of the U.S., European stocks advanced, despite signs of slowing economic growth, a near-recession in Germany, and Brexit-related uncertainty in the U.K. that remained unresolved until the very last month of the year. (“Brexit” – an abbreviation for “British exit” – refers to the United Kingdom’s referendum to leave the European Union.) Emerging-market stocks advanced, overcoming bouts of volatility tied to global trade tensions, China’s economic slowdown, and a strong U.S. dollar.

For the year ended December 31, 2019, Fund holdings in the communication services and financials sectors boosted the Fund’s relative returns, with Amsterdam-based Altice Europe among the top contributors thanks to better-than-expected growth. One of the biggest detractors from relative returns was health-care sector stock Teva Pharmaceutical Industries, whose stock price suffered amid continued opioid litigation.

Capital Research’s outlook

Fund management continues to monitor volatility and potential headwinds brought about by political and trade uncertainty. Given how late it may be in the current bull market, they seek opportunities in particular within those regions, countries, and sectors less affected by these global headlines. Their focus on a company’s fundamental growth drivers and intrinsic worth is critical to helping them identify investments that they believe represent the best value over the long term.

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GROWTH OF $10,000 INVESTMENT - SERVICE CLASS I

The graph above illustrates the Fund’s historical performance for the past 10 fiscal years or since inception (for Funds lacking 10-year records) in comparison to its benchmark index, as well as one or more additional indexes, if applicable.

* Benchmark

Generally accepted accounting principles require adjustments to be made to the net assets of the Fund at period end for financial reporting purposes only, and as such, the total return based on the unadjusted net asset value per share may differ from the total return reported in the financial highlights.

Performance results in the graph and table reflect the percentage change in net asset value, including reinvestment of dividends and capital gains distributions. Investors should note that the Fund is a professionally managed mutual fund, while the index or indexes shown above are unmanaged, cannot be purchased directly, and, with the exception of any peer group index, do not incur expenses.

Performance data quoted in the graph and table represents past performance; past performance is not predictive of future results. The investment return and principal value of shares of the Fund fluctuate, so your shares, when sold, may be worth more or less than their original cost. Current performance of the Fund may be lower or higher than the performance quoted. Performance data current to the most recent month end may be obtained by calling 1-888-309-3539.

Investors should read the Fund’s prospectus with regard to the Fund’s investment objectives, risks, and charges and expenses in conjunction with these financial statements. The Fund’s return reflects changes in the net asset value per share without the deduction of any product charges (e.g., cost of insurance, mortality and expense risk charges, administrative fees, and CDSL). The inclusion of these charges would have reduced the performance shown here.

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What is the investment approach of MML American Funds Core Allocation Fund, and who is the Fund’s investment adviser?

The Fund seeks to achieve as high a total return over time as is considered consistent with prudent investment risk, preservation of capital, and recognition of the Fund’s stated asset allocation. The Fund is a “fund of funds” and seeks to achieve its investment objective by principally investing in a combination of series of the American Funds Insurance Series (the “American Underlying Funds”), managed by Capital Research and Management Company (Capital Research), using a flexible asset allocation approach. The Fund’s investment adviser invests the Fund’s assets in a combination of U.S. domestic and international American Underlying Funds. As of the date of this report, it is expected that the American Underlying Funds will include Class 1 shares of the American Funds Insurance Series – Bond Fund, the American Funds Insurance Series – Blue Chip Income and Growth Fund, the American Funds Insurance Series – Growth-Income Fund, and the American Funds Insurance Series – International Fund. The Fund’s investment adviser allocates the Fund’s assets among a variety of different asset classes through investing in American Underlying Funds in response to changing market, economic, and investment conditions. The Fund’s investment adviser is MML Investment Advisers, LLC (MML Advisers).

How did the Fund perform during the 12 months ended December 31, 2019?

The Fund returned 18.12%, substantially underperforming the 31.49% return of the S&P 500^® Index (the “benchmark”), which measures the performance of 500 widely held stocks in the U.S. equity market. The Fund underperformed the 21.51% return of the MSCI All Country World Index (ACWI) ex USA, which measures the performance of the large- and mid-cap segments of the particular regions, excluding U.S. equity securities, including developed and emerging markets. The Fund outperformed the 8.72% return of the Bloomberg Barclays U.S. Aggregate Bond Index, which measures the performance of the investment grade, U.S. dollar-denominated, fixed-rate taxable bond market, including Treasuries, government-related and corporate securities, mortgage-backed securities (MBS) (agency fixed-rate and hybrid ARM pass-throughs), asset-backed securities (ABS), and commercial mortgage-backed securities (CMBS). Finally, the Fund’s 18.12% return underperformed the 22.29% return of the Custom MML Core Allocation Index, which comprises the benchmark (55%), the Bloomberg Barclays U.S. Aggregate Bond Index (35%), and the MSCI ACWI ex USA (10%).

For a discussion on the economic and market environment during the 12-month period ended December 31, 2019, please see the Economic and Market Review, beginning on page 3.

Capital Research’s discussion of factors that contributed to the Fund’s performance

The following paragraphs discuss the results of each of the Fund’s underlying portfolio components. Because the Fund invests in both stock- and bond-based portfolios, its overall performance may vary considerably from the broad market indexes listed above, which each measure the performance of a specific group of securities. These component-specific discussions focus on results and positioning versus each component’s main comparative index, not how the Fund as a whole performed or was positioned versus those indexes.

TheAmerican Funds Insurance Series –Bond Fund (the “Bond Fund”) outperformed the Bloomberg Barclays U.S. Aggregate Bond Index (the “Index”) for the year, as the Bond Fund benefited from active duration management as interest rates declined over the period. (Duration is a measure of a bond fund’s sensitivity to interest rates. The longer the duration, the greater the price impact on the bond or portfolio when interest rates rise or fall.) Sector selection was also a positive factor, as larger-than-Index investments in Treasury Inflation Protected Securities (TIPS), investment-grade corporate bonds, high yield, and emerging markets all contributed to full-year performance. Security selection within agency mortgage backed pass-through securities was the primary detractor from the Bond Fund’s relative results for the period.

TheAmerican Funds Insurance Series –Blue Chip Income and Growth Fund (the “Income and Growth Fund”) underperformed the benchmark in 2019, despite U.S. equities having had their strongest annual gains since 2013, ending the year near all-time highs. Performance was mixed within the Income and Growth Fund holdings in the health care sector. Specifically, pharmaceutical companies such as AbbVie and Teva Pharmaceutical hurt results relative to the benchmark. However, UnitedHealth Group was among the top contributors to relative returns within health care, which consistently surpassed earnings estimates each quarter throughout the year. Another top contributor to the Income and Growth Fund’s performance was Qualcomm, where an improved relationship with Apple and the prospect of a 5G wireless network bolstered both earnings prospects and the company’s valuation.

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TheAmerican Funds Insurance Series –Growth-Income Fund (the “Growth-Income Fund”) underperformed the benchmark for the year, as the Growth-Income Fund’s underweight position, relative to the benchmark, in information technology stocks, a sector that performed strongly over the period, weighed on relative returns. In particular, an underweight allocation to technology leader Apple hindered results the most, as Apple shares outperformed the broader market over the period. Growth-Income Fund holdings in the industrials sector contributed the most to relative returns. Among industrials, aircraft component manufacturer TransDigm Group was a top contributor.

TheAmerican Funds Insurance Series –International Fund (the “International Fund”) outperformed the MSCI ACWI ex USA (the “Index”) for the year, as global stocks generated the best returns in a decade in 2019, boosted by aggressive central bank stimulus measures and easing trade tensions toward the end of the year. The International Fund’s holdings in the communication services and financials sectors boosted relative returns, with Amsterdam-based Altice Europe among the top contributors thanks to better-than-expected growth. One of the biggest detractors from relative returns was health care sector stock Teva Pharmaceutical Industries, whose stock price suffered amid continued opioid litigation.

Capital Research’s outlook

In the near term, Fund management is aware that the market is keeping a close watch on U.S. monetary policy, U.S. trade relationships, the current shape of the yield curve, and the implications of those three factors on the U.S. economy. Fund management continues to favor well-managed, high-quality companies that are capable of paying dividends in various economic environments. Fund management remains optimistic that this focus, supported by their global research, will help them identify attractive long-term investment opportunities.

On the international stage, Fund management continues to monitor volatility and potential headwinds brought about by political and trade uncertainty. Given how late Fund management believes it may be in the current bull market, they seek opportunities in particular within those regions, countries, and sectors less affected by these global headlines. Fund management believes their focus on a company’s fundamental growth drivers and intrinsic worth is critical to helping them identify investments that they believe represent the best value over the long term.

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GROWTH OF $10,000 INVESTMENT - SERVICE CLASS I

The graph above illustrates the Fund’s historical performance for the past 10 fiscal years or since inception (for Funds lacking 10-year records) in comparison to its benchmark index, as well as one or more additional indexes, if applicable.

* Benchmark

Generally accepted accounting principles require adjustments to be made to the net assets of the Fund at period end for financial reporting purposes only, and as such, the total return based on the unadjusted net asset value per share may differ from the total return reported in the financial highlights.

Performance results in the graph and table reflect the percentage change in net asset value, including reinvestment of dividends and capital gains distributions. Investors should note that the Fund is a professionally managed mutual fund, while the index or indexes shown above are unmanaged, cannot be purchased directly, and, with the exception of any peer group index, do not incur expenses.

Performance data quoted in the graph and table represents past performance; past performance is not predictive of future results. The investment return and principal value of shares of the Fund fluctuate, so your shares, when sold, may be worth more or less than their original cost. Current performance of the Fund may be lower or higher than the performance quoted. Performance data current to the most recent month end may be obtained by calling 1-888-309-3539.

Investors should read the Fund’s prospectus with regard to the Fund’s investment objectives, risks, and charges and expenses in conjunction with these financial statements. The Fund’s return reflects changes in the net asset value per share without the deduction of any product charges (e.g., cost of insurance, mortality and expense risk charges, administrative fees, and CDSL). The inclusion of these charges would have reduced the performance shown here.

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The accompanying notes are an integral part of the financial statements.

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Statements of Assets and Liabilities

December 31, 2019

The accompanying notes are an integral part of the financial statements.

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